Mass spectrometers are an apparatus for ionizing molecules and atoms of a component included in a gaseous, liquid or solid sample and separating the ions according to their mass-to-charge ratio to detect them in order to identify the component or determine its content. These days, it is widely used for various purposes such as the determination of biological samples or analysis of proteins or peptides.
In the fields of biochemistry and medicine, which treat living organisms, there is a great demand for obtaining the distribution information of proteins included in a cell in vivo without destroying the cell. In order to meet such a demand, a mass microscope having both the function of a microscope and that of a mass spectrometer has been developed in many places. A mass microscope makes it possible to obtain information about the distribution or other properties of a substance in a two-dimensional area on a sample set on a preparation or the like. This type of mass analysis conventionally requires repeating the operations of two-dimensionally scanning the irradiation point of an ionization laser beam or particle beam on the sample, collecting the ions generated from the irradiation point every time the irradiation point is shifted, mass-separating the ions and eventually detecting the separated ions. Repeating such operations requires a considerably long period of time if the mass analysis needs to be performed over a two-dimensional region with a certain area. This situation is undesirable not only because the analysis time becomes long, but because the biological sample may be damaged or degraded during the long period of time, causing the analysis to be rather inaccurate.
To address this problem, a method has been proposed in Non-Patent Document 1. According to this method, ions are two-dimensionally generated so that they reflect the two-dimensional distribution of the substances on a sample. The resultant ions are then mass-separated by a time-of-flight (TOF) mass separator and detected by a two-dimensional detector. Unfortunately, this method is costly since arranging a plurality of conventional ion detectors in a two-dimensional pattern requires providing as many measurement circuits (amplifiers, digitizers and so on) in parallel. On the other hand, decreasing the number of ion detectors for cost reduction deteriorates the positional (or spatial) resolution of the system, making the method rather impractical.
As a means for overcoming such problems, the inventors have proposed a novel mass microscope in Japanese Patent Application No. 2006-58816. The new mass microscope employs an image sensor having a special construction, called the in-situ storage image sensor, as the two-dimensional array detector. In this mass microscope, ions are mass-separated by a TOF mass separator or similar device and then directed to a micro channel plate (MCP), which emits electrons by an amount larger than that of the incident ions. Those electrons are converted into light by a fluorescent plate, and this light is further converted into an electric signal by the in-situ storage image sensor. Thus, an electric signal corresponding to the amount of the original ions is extracted.
Detailed information concerning the in-situ storage image sensor is available in the existing literature, such as Patent Document 1 or 2. Accordingly, no detailed description is hereby provided. Briefly, an in-situ storage image sensor includes storage charge-coupled devices (CCDs), each of which is coupled with a photodiode used as a light-receiving element and is capable of storing and transferring signals for a predetermined number of records (or frames). During an image-capturing process, the pixel signals generated by photoelectric conversion at the photodiode are sequentially transferred to the storage CCD. After the image-capturing process is completed, the pixel signals corresponding to the predetermined number of recorded images, which have been stored in the storage CCD, are collectively read out from the sensor to externally reproduce the predetermined number of images. Pixel signals that have exceeded the predetermined number of records during the image-capturing process are chronologically discarded from the oldest one. Thus, a predetermined number of the latest pixel signals are constantly held in the storage CCD. When the transfer of pixel signals to the storage CCD is discontinued at the end of the image-capturing process, the latest set of images from the newest image back through the predetermined number of images can be retrospectively obtained. Thus, as compared with normal image sensors that require extracting image signals corresponding to one frame every time those image signals are obtained, the in-situ storage image sensor is characterized in that it can repeatedly capture images at extremely high rates.
Although this type of two-dimensional array detector can acquire images at extremely high rates, the number of images that can be acquired is structurally limited. For example, if a detector capable of acquiring 100 frames of images at a rate of one million frames per second is used, it is possible to obtain mass analysis data over a time range of 100 μsec at intervals of 1 μsec. If a detector capable of acquiring 100 frames of images at a higher rate of ten million frames per second is used, it is possible to obtain mass analysis data over a time range of 10 μsec at intervals of 100 nsec. In any case, the number of mass analysis data is limited by the number of frames that the two-dimensional array detector can successively acquire.
In a TOF mass separator, the mass-to-charge ratio difference is represented as a difference in the flight time. Therefore, in order to improve the mass resolution, the time interval at which the acquisition of mass analysis data is repeated should preferably be as short as possible. Meanwhile, the range of the mass-to-charge ratios that can be measured by one analysis operation should preferably be as wide as possible. For that purpose, the acquisition of mass analysis data must be repeated as many times as possible.
For example, if an ion with a mass-to-charge ratio of 1000 [amu] is given 10 [keV] of energy and made to fly straight for a distance of 2 [m], its flight time will be approximately 45.69 μsec, whereas the flight time of an ion with a mass-to-charge ratio of 1010 [amu] under the same conditions will be approximately 45.92 μsec. The flight-time difference between these two ions is approximately 0.23 μsec. This example demonstrates that the mass-to-charge ratio difference of 10 [amu] is detectable if the mass analysis data can be repeatedly acquired at intervals of 0.2 μsec. Under the present conditions, using a sensor capable of acquiring 100 frames of images enables the mass analysis data to be repeatedly collected for 0.2×100=20 μsec. For example, if the acquisition of mass analysis data is initiated at a point in time (t1) where 45 μsec have elapsed since an emission of ions, the data acquisition can be continued until the point in time (t2) where the lapse of time from the emission of ions reaches 65 μsec. Meanwhile, the flight time of an ion with a mass-to-charge ratio of 2000 [amu] is approximately 64.61 μsec. Thus, the mass-to-charge ratios that can be measured by the data acquisition over the time range from t1 to t2 are limited to the range of 1000 to 2000 [amu].
The flight time of an ion with a mass-to-charge ratio of 10000 [amu] is 144.47 μsec. Accordingly, the difference in the flight time between 1000 [amu] and 10000 [amu] is 100 μsec. Measuring this flight-time range with a two-dimensional array detector capable of simultaneously acquiring 1000 frames results in 1 μsec of time difference per frame (i.e. the time resolution). As stated previously, the flight time of an ion with 1000 [amu] is approximately 45.69 μsec; therefore, the point in time later than that by the time resolution 1 μsec is 46.69 μsec. Detected at this point in time is an ion with a mass-to-charge ratio of 1044 [amu]. Accordingly, the mass resolution of this mass spectrometer is no shorter than approximately 44 [amu].
Thus, the conventional mass microscope has the problem that increasing the mass resolution narrows the mass-to-charge ratio range that can be covered by one measurement, whereas widening the mass-to-charge ratio range lowers the mass resolution. It is theoretically possible to simultaneously achieve both a wider mass-to-charge ratio range and higher mass resolution by using a two-dimensional array detector capable of acquiring a larger number of frames. However, for that purpose it is necessary to increase the area of the storage CCD mounted on the device, which will correspondingly reduce the area of the photodiode and deteriorate the sensitivity or spatial resolution of the detector.
Patent Document 1: Japanese Unexamined Patent Application Publication No. 2001-345411
Patent Document 2: Japanese Unexamined Patent Application Publication No. 2004-235621
Non-Patent Document 1: Yasuhide NAITO, “Seitai Shiryou Wo Taishou Ni Shita Shitsuryou Kenbikyou (Mass Microscope for Bio-samples)”, J. Mass Spectrom., Soc. Jpn., Vol. 53, No. 3, 2005